This tale is older then the legend of Nightmare Moon, older than the legend of Discord, even older than Equestria itself.
Over ten thousand years before the return of Nightmare Moon an evil was sealed away and forgotten by all exept those that gaurd its prison.
This tale is of a young filly who's family has recently suffered a terrible loss. And of a young colt who was not born by natural means. It is a tale of Necromancers and those who seek to destroy them. It is a tale of tragedy.
This is how the entity known as Grenth came to be, and how an innocent filly, become one of the most powerful and evil Lich the world has ever seen.